Steam flow in a steam turbine has a wetness of at least 8% near the last stage turbine. The steam flow generates water drops, and the wet steam flow may lead to a moisture loss, and the turbine efficiency may be reduced. In addition, the water drops generated from the wet steam flow may collide with a rotor blade rotating at a high speed, which may lead to erosion. The water drops contained in the wet steam flow attach on a surface of a stator blade to from a water film. The water film is forced by the wet steam flow to form a water film flow, and the water film flow flows to the trailing edge side of the stator blade. Then, the water film flow may break at the trailing edge of the stator blade and form coarse water drops on a downstream side of the stator blade. The coarse water drops may be one of the greatest reasons that cause erosion of the rotor blade.
FIG. 16 is a diagram illustrating a flow field of a steam flow of a steam turbine. A stator blade 100 is disposed between and connected to a diaphragm 104 provided on a rotor shaft (not shown) side and a support ring 106 provided on a tip side. Small water drops dw contained in a wet steam flow s attach onto a surface of the stator blade 100, particularly onto a pressure surface fs of the stator blade, which faces to more amount of wet steam flow s than a suction surface bs of the stator blade, and the water drops collect on a surface of the stator blade to form a water film flow sw moving toward the trailing edge side of the stator blade. The water film flow sw on the surface of the stator blade flows from the leading edge fe side of the stator blade to the trailing edge re side of the stator blade, and it breaks into coarse water drops cw at the trailing edge re of the stator blade. The coarse water drops cw collide with a rotor blade on a downstream side to erode a surface of the rotor blade.
FIG. 17 is a diagram illustrating a velocity triangle of a wet steam flow s at the outlet of the stator blade. An absolute velocity Vcw of a coarse water drop cw is smaller than an absolute velocity Vs of the wet steam flow s on the outlet portion of the stator blade. Accordingly, in the relative velocity field considering the circumferential velocity U of the rotor blade 102, the coarse water drop cw has a relative velocity Wcw which is larger than the relative velocity Ws of the wet steam flow s and has a smaller incident angle, and it collides with a surface of the rotor blade 102 at a high speed. Thus, the rotor blade 102 is susceptible to erosion by the coarse water drops cw, particularly near the tip of the blade where the circumferential velocity is relatively large. Further, the collision of the coarse water drops cw may lead to increase in breaking loss of the rotor blade 102.
In view of this, in order to remove water drops on a surface of a rotor blade, such a method is conventionally employed that a slit opening to a surface of a stator blade is formed to introduce the water drops on the surface of the stator blade from the slit, thereby to remove the water drops from the flow field of the steam flow. Each of JP H64-080705 A and JP H09-025803 A discloses a structure of a stator blade having such a slit formed.
FIG. 18 to FIG. 21 are diagrams of an example of a stator blade having such a slit formed. In FIG. 18, the both ends in the axial direction of the stator blade 100 are connected to a diaphragm 104 which has a separated body from a rotor shaft 108 and which is provided on the rotor shaft 108 side, and a support ring 106 on a tip side, respectively. The rotor blade 102 is integrally formed with the rotor shaft 108 via a disk rotor 110. Plurality of slits 112 and plurality of slits 114, extending along the axial direction of the stator blade 100, are formed on the pressure surface fs and the suction surface bs of the stator blade, respectively. Inside the support ring 106, a hollow portion 106a is formed.
As shown in FIG. 19 and FIG. 20, a hollow portion 100a is formed inside the stator blade 100. The hollow portion 100a is in communication with the hollow portion 106a via a hole 106b formed in the support ring 106. The hollow portion 100a is in communication with a low pressure region via a hole 106c. The water film flow sw on the surface of the stator blade and flowing toward the trailing edge is drawn through the slits 112 and 114 into the hollow portion 100a. A slit groove 116 is formed at a back end of the support ring 106, and the slit groove 116 is in communication with the low pressure region. The low pressure region has a relatively low pressure than the flow field of the steam flow such that the water film flow sw can be drawn through the slits 112 and slits 114 and discharged to the hollow portion 106a. 
FIG. 20 is a diagram illustrating a conventional example having a slit 112 opening to the pressure surface of the stator blade. The water film flow sw formed on the pressure surface fs of the stator blade collects water drops and the collection amount of the water drops becomes larger as the water film flow moves from the leading edge fe of the stator blade to the trailing edge re of the stator blade. Thus, in order to increase the water removal amount, the slits opening to the pressure surface fs of the stator blade are formed at the most trailing edge side of the stator blade in such a range that communication between the slits 112 and the hollow portion 100a is possible.
Further, as shown in FIG. 21, the stator blade trailing edge side wall surface 112a and the stator blade leading edge side wall surface 112b of the slit 112, which is formed on the pressure surface fs of the stator blade according to the conventional technique, are formed so as to have an inclination angle A of larger than 90°, to the leading edge side reference plane of the pressure surface fs of the stator blade, as disclosed in JP H64-080705 A. This is because, by making the widths of the inlet opening e and the outlet opening f of the slit 112 larger than the slit width h of the slit 112, and by permitting the slit 112 to face the flow direction of the wet steam flow s, the wet steam flow s becomes likely to move into the slit. Thus, the wet steam flow s may be actively drawn into the slit 112, and the water film flow sw may be drawn along with the wet steam flow s into the slit 112.